Our previous studies of genetic obesities indicate that there is a competition for available nutrients between muscle and adipose tissue when the obese animals are pair-fed to lean controls. In effect, adipose cells are preferentially utilizing energy for fat synthesis over muscle tissue use for protein synthesis. Studies described here are designed to determined in neonatal development or during obesity onset, the causative factors in spontaneous obesities. Our main objectives are as follows: (1) to determine the effect of endocrinectomies and replacement hormone therapy on the development of spontaneous obesities and the ontogenesis of enzyme profiles which are normally found in obese animals and man, (2) to determine in energy required for maintenance and BMR is lower in obese animals of various etiologies (i.e., VMH, hyperphagic, neonatal overnutritional, genetic, fa/fa, ay/a, and ob/ob, (3) to determine partial efficiencies of energy deposited as protein and fat in lean and obese animals, (4) to compare relative metabolic costs of protein turnover and futile cycles in lean and obese animals, (5) to determine which factors regulate the competition of muscle and adipose tissue for available nutrients in lean and obese Zucker rats (i.e., blood flow distribution, transport, enzyme Km), and (6) to characterize the development of genetic obesity in Gallus domesticus.